The electric power steering apparatus is a machine which drives the motor to generate the steering assist force according to the steering torque applied by the driver. Recently, such electric power steering apparatus have been faced with a demand to operate on bulk power. To meet this demand, an electric power steering apparatus has been proposed in which an auxiliary power supply (capacitor) independent from a battery is connected in series with the battery as needed so that a large amount of power may be supplied at high voltage (see, for example, Patent Document 1). By the way, if the battery breaks down (fails), the electric power steering apparatus suddenly fails to fulfill its steering assist function. For the sake of improvement of reliability, therefore, there has also been suggested potential for using the auxiliary power supply as a backup power source such as to ensure that the battery breakdown does not entail sudden loss of the steering assist force.
In a case where the auxiliary power supply is used as the backup power source, a control source voltage (Vcc) is also supplied from the auxiliary power supply. Therefore, the auxiliary power supply must save such an amount of charge as to be able to maintain the control source voltage even though the auxiliary power supply is designed to supply the stored energy to the motor of the electric power steering apparatus. Namely, the auxiliary power supply has a lower voltage limit at which the auxiliary power supply reaches a power discharge limit. If the motor freely draws power from the auxiliary power supply to the power discharge limit, the discharge ends as soon as the auxiliary power supply reaches the power discharge limit. This results in the sudden loss of the steering assist force. In this case, the driver experiences a feeling of strangeness associated with an abrupt increase in steering load.